1. Field of the Invention
The present invention relates to an implantable stent for transluminal implantation in body lumen, especially found in peripheral and coronary blood vessels, but also for use in bile ducts, urethras or ileums.
2. Description of the Related Art
There are several designs of stents, permanently implantable devices, for transluminal insertion into blood vessels and other lumen to prevent or reverse occlusion thereof. There are three basic categories of device, namely heat-expandable devices, balloon-expandable device and self-expanding devices. The present invention is concerned with self-expanding devices with an optional heat expanding capability, that is which are inserted into the body lumen in a radially compressed condition and which are mechanically biased towards a radially expanded position. Upon being released in the blood vessel at the desired position, the stent expands radially exerting outwardly directed pressure upon the inner surface of the wall of the body lumen in which it is positioned.
One such expanding device which is commercially available is the so-called Wallstent. The device is described in WO-A-83/03752. It consists of two sets of counter-rotating helical filaments of metallic wire which are braided together in a one over/one under pattern. Although it is suggested in the above mentioned patent specification that the axially directed angle α between filaments at crossover points can be acute, it is preferably at least 900 and more preferably 1000 or more, which the author indicates is essential to provide adequate radial strength in use such that the stent remains in its radially expanded condition.
There are difficulties with the braided stent of this type. One difficulty is that with a high angle α the change in axial length between the radially expanded and the radially compressed condition is generally high. One way of overcoming that problem has been described, in WO-A-92/00043, which describes a stent consisting of two co-axial and slidably connected braided stent segments. In the radially compressed condition the stent segments are in a telescoped condition such that the stent portions overlap with one another over a higher proportion of their respective lengths. Upon deployment each segment contracts in the axial direction, such that the length of overlap between the sections is decreased whilst retaining the same distance between the stent ends. This arrangement, however, adds complexity to the manufacture and deployment and it is difficult to allow for the mutual sliding between stent segments without snagging.
Another difficulty with braided stents in general is the tendency of the filaments at the end of the stent to unravel and splay outwards before or after deployment. This tendency makes the stent difficult to handle and the splayed ends can damage the inside wall of the body vessel in which the stent is deployed. In WO-A-83/03752, it is suggested that the filaments may be joined to one another at the end of the stent. However, as explained in a later specification by Wallsten et al in U.S. Pat. No. 5,061,275, for stents with a high angle α between counter-rotating filaments, that this rigidifies the ends of the prosthesis. This makes it difficult to compress the stent into its radially compressed condition. It is also said to accentuate the risk of penetration of the ends of the filaments through the wall of the vessel.
In DE-A-4,240,177 & U.S. Pat. No. 5,503,636, a braided stent is formed from wire having a rectangular cross-section. The wires are rigidly connected to one another at the crossover points and at the ends of the stent, by sheathing the filaments with a plastics material, applied whilst the stent is held in its radially expanded condition, in which the angle α is obtuse.
An early disclosure of a dilator for body vessels appears in GB-A-1205743. This consists of counter-rotating helices which may be braided or may not be braided but joined at the crossover points. Where the dilator is made from counter rotating wire helices, it is suggested that the ends of the metal wires may be joined by swaging or welding. The angle between filaments at the crossover is said to be preferably in the range 45° and 60°, which is said to give a desired low ratio of axial extension to change in diameter (i.e. length change upon change in radius).
Another example of a braided stent is described in EP-A-0,183,372. The filaments in this publication are formed preferably of plastics material and the device generally has a warp thread extending parallel to the axis of the stent. The warp filament may be formed of shrinkable plastics to allow expansion of the stent insitu to its radially expanded condition.